Ophiactis savignyi

Lore & Background

Ophiactis savignyi was first described in 1842 by the German zoologists Johannes Peter Müller and Franz Hermann Troschel. The specific name honors the French zoologist Marie Jules César Savigny. The disc is up to 5 mm in diameter, with a pale greenish-brown aboral surface covered by large overlapping scales and a scattering of spines. The six arms are long, slender, and tapering, each segment bearing five or six thorny spines. The oral surface is cream-colored.

This brittle star has a near cosmopolitan distribution in warm seas, occurring in the western Indo-Pacific, eastern Pacific, and on both sides of the Atlantic. Populations in the Pacific and Atlantic were only united after the Panama Canal was cut in 1914. It is found from the intertidal zone down to about 500 meters, in reefs, mangroves, seagrass meadows, among seaweeds, and in contaminated waters, often living inside sponges possibly as a commensal.

O. savignyi is a deposit feeder and scavenger, feeding on detritus. It raises arm tips to detect food, rolls nutritious items into a ball, and moves them to the mouth using tube feet. Stomach contents have included bryozoans, foraminiferans, and gastropods among detritus and sand grains. Individuals are either male or female and can reproduce sexually or asexually by fragmentation. In Taiwan, mature gametes occur from March to December, with peak spawning in May and June; fission occurs mostly between July and December. Sampling found 48% of individuals were regenerating, and the sex ratio was 24 males per female.

Reader's Guide

Ophiactis savignyi is notable as the brittle star with the most widespread distribution, found in tropical and subtropical waters across all major oceans. Its ability to reproduce both sexually and asexually by fragmentation contributes to its success. Asexual reproduction allows small individuals with six arms to split and regenerate, often resulting in five arms. Larger individuals also fragment, with males doing so more often than females, which may explain the excess of males in populations. In Taiwan, the sex ratio is heavily skewed at 24 males per female. The species is often found living inside sponges, such as Lissodendoryx schmidti in Hawaii, where up to 20 individuals clump together in cavities filled with sponge slime. These brittle stars frequently have broken or partly regenerated arms, though how they enter the sponge or feed and reproduce in that environment is not clear. Its presence in contaminated waters and diverse habitats underscores its adaptability.

Did You Know?

• It was first described in 1842 by German zoologists Johannes Peter Müller and Franz Hermann Troschel.
• In Taiwan, the sex ratio is 24 males for every one female.
• Up to 20 individuals can be found clumped together in cavities in the sponge Lissodendoryx schmidti in Hawaii.